Hot-melt sealing glass compositions and methods of making and using the same

ABSTRACT

Hot-melt sealing glass compositions that include one or more glass frits dispersed in a polymeric binder system. The polymeric binder system is a solid at room temperature, but melts at a temperature of from about 35° C. to about 90° C., thereby forming a flowable liquid dispersion that can be applied to a substrate (e.g., a cap wafer and/or a device wafer of a MEMS device) by screen printing. Hot-melt sealing glass compositions according to the invention rapidly re-solidify and adhere to the substrate after being deposited by screen printing. Thus, they do not tend to spread out as much as conventional solvent-based glass frit bonding pastes after screen printing. And, because hot-melt sealing glass compositions according to the invention are not solvent-based systems, they do not need to be force dried after deposition.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit from U.S. Provisional PatentApplication Ser. No. 61/081,051 filed 16 Jul. 2008, which isincorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to hot-melt sealing glass compositions andmethods of making and using the same.

2. Description of Related Art

FIG. 1 shows a schematic cross-section view of an exemplarymicroelectromechanical systems (“MEMS”) device 10 formed in or on adevice wafer 20 made of silicon or glass. The MEMS device 10 could be anaccelerometer, rate sensor, actuator, pressure sensor etc. Signal lines30, a portion of which may be formed in the device wafer 20,electrically connect the MEMS device 10 to a microprocessor and/or toother circuitry (not shown). A cap wafer 40 made of silicon or glass isbonded to the device wafer 20 using a sealing glass composition, whichis melted and re-solidified to form a hermetic glass seal 50 between thecap wafer 40 and the device wafer 20. The cap wafer 40, the hermeticglass seal 50 and the device wafer 20 thus cooperate to define a packagecomprising a cavity 60 within which the MEMS device 10 is enclosed andprotected.

By their very nature of operation, MEMS devices must be free to move tosome degree. Thus, the seal between the device wafer and the cap wafermust prevent dust, moisture and other foreign matter that couldinterfere with the function of the MEMS device from entering the cavity.Some MEMS devices, such as pressure sensors, for example, require thatthe cavity be completely evacuated and hermetically sealed. Some MEMSdevices, such as motion sensors and accelerometers with resonatingmicromachined components for example, operate more effectively in avacuum. And some MEMS devices need a gas back-filled to create a certainatmosphere. A hermetical seal between the cap wafer and the device waferalso ensures that moisture is excluded from the cavity, which could leadto the formation of ice crystals at low temperatures and/or otherwiseimpede the operation of the MEMS device.

Sealing glass compositions used in MEMS device fabrication are typicallyapplied using screen printing techniques, in which the sealing glasscomposition is deposited in the form of a paste that contains aparticulate glass frit material, a thixotropic binder, and a solvent forthe binder. The proportions of glass frit, binder and solvent areadjusted to allow screen printing of a controlled volume of the paste ona designated bonding surface of one of the wafers, typically on the capwafer. After drying, burning out the binder (BBO) and pre-glazing, whichremoves all of the organic components from the glass frit bonding paste,the cap and device wafers are aligned and then mated so that the glassfrit particles contact complimentary bonding surfaces. The wafers arethen incrementally heated to remelt, flow and impart wetting of thewafer surfaces by the glass frit so that upon cooling, the glass fritmaterial re-solidifies to form a substantially homogeneous glass bondline between the wafers.

Because the glass frit particles in glass frit bonding pastes aredispersed in a binder and solvent system, the glass frit particles havea tendency to flow out (i.e., spread) somewhat after they are screenprinted and until the solvent is removed. For example, a screen printedline of glass frit bonding paste having an initial line width of about160 microns will typically spread out to a width of about 200 micronsbefore the solvent is removed from the paste. It would be advantageousif the initial line width could be maintained and if the drying stepcould be eliminated.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, the present invention is directed to hot-meltsealing glass compositions and methods of making and using the same.Hot-melt sealing glass compositions according to the invention compriseone or more glass frits dispersed in a polymeric binder system. Thepolymeric binder system is a solid at room temperature (herein presentlydefined as being about 22.5° C.), but melts at a temperature of fromabout 35° C. to about 90° C., thereby forming a flowable liquiddispersion that can be applied to a substrate (e.g., a cap wafer and/ora device wafer) by screen printing. Hot-melt sealing glass compositionsaccording to the invention rapidly re-solidify and adhere to thesubstrate after being deposited by screen printing. Thus, they do nottend to spread out as much as conventional solvent-based glass fitbonding pastes after screen printing. And, because hot-melt sealingglass compositions according to the invention are not solvent-basedsystems, they do not need to be force dried after deposition. There-solidified screen printed sealing glass pattern can withstandrigorous part handling without detaching from the substrate.

As noted above, the hot-melt sealing glass compositions according to theinvention comprise one or more glass frits, which are dispersed in apolymeric binder system. Preferably, the least amount of polymericbinder system necessary to obtain adequate flow in the liquid dispersionstate is used. In one embodiment of the invention, hot-melt sealingglass compositions according to the invention comprise about 85% byweight of one or more glass fits and about 15% by weight of thepolymeric binder system.

The glass frit or combination of glass frits in the hot-melt sealingglass compositions according to the invention melt and flow atrelatively low firing temperatures (e.g., 500° C. or less, preferably480° C. or less, more preferably 450° C. or less). In preferredlow-temperature firing regimes, the hot-melt sealing glass compositionof the invention is free of intentionally added SiO₂ and/or free ofintentionally added Al₂O₃. Preferably, the polymeric binder systemcompletely burns out below the glass transition temperature of the glassfrit or combination of glass frits. It is envisioned that the glasscomponent may include combinations of two or more glass frits disclosedherein. A polymeric binder system comprising a major amount of a fattyalcohol that is a waxy solid at room temperature and minor amounts ofone or more polyacrylate polymers is presently preferred.

An embodiment of the invention is a screen-printable hot-melt sealingglass composition comprising:

-   -   from about 50% by weight to about 95% by weight of a glass        component comprising at one or more glass frits, and    -   from about 5% by weight to about 50% by weight of a polymeric        binder system that is a solid at 22.5° C. but which melts at a        temperature within the range of from about 35° C. to about 90°        C.,

-   wherein the polymeric binder system is capable of complete burn-out    at a temperature below about 450° C., and

-   wherein the glass component is capable of melting and flowing at a    temperature below about 500° C.

Another embodiment of the invention is a method of bonding a cap waferto a device wafer so as to hermetically seal and isolate a MEMS devicein a cavity defined therebetween, the method comprising:

-   -   providing a hot-melt sealing glass composition comprising:        -   from about 50% by weight to about 95% by weight of a glass            component comprising one or more glass frits, and        -   from about 5% by weight to about 50% by weight of a            polymeric binder system that is a solid at 22.5° C. but            which melts at a temperature within the range of from about            35° C. to about 90° C.,        -   wherein the polymeric binder system is capable of complete            burn-out at a temperature below about 450° C., and        -   wherein the glass component is capable of melting and            flowing at a temperature below about 500° C.    -   heating the hot-melt sealing glass composition above the melting        point of the polymeric binder system to form a molten paste;    -   depositing the molten paste onto the cap wafer and/or the device        wafer by screen printing;    -   allowing the deposited molten paste to re-solidify;    -   positioning the cap wafer and device wafer in relation to each        other such that the re-solidified hot-melt sealing glass        composition is positioned therebetween; and    -   heating the cap wafer and device wafer to a temperature above        the melting point of the glass component to form a hermetic seal        between the cap wafer and device wafer that isolates the MEMS        device in the cavity defined therebetween.

The foregoing and other features of the invention are hereinafter morefully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section view of a MEMS device formed on adevice wafer and covered by a cap wafer.

DETAILED DESCRIPTION OF THE INVENTION

The hot-melt sealing glass composition according to the inventioncomprises one or more glass frits dispersed in a polymeric bindersystem. The one or more glass frits are preferably low-melting glassfrits such as those glass frits which are conventionally used in sealingglass applications. The loading of the one or more glass frits ispreferably as high as possible while still being able to apply thematerial using conventional screen printing techniques. Loadings of 50%to about 95% by weight are typically achievable.

The particles of the one or more glass frits must be small in order tobe applied using screen printing technology. Average particle sizes ofabout 3 to about 10 microns, and more preferably about 5 microns, arepreferred. It will be appreciated that particle sizes can be adjusted,as necessary, to facilitate specific screen-printing demands.

When the hot-melt sealing glass composition is used for sealing a capwafer to a device wafer during the fabrication of a MEMS device, theglass frit or the combination of two or more glass fits preferably hasthe following cumulative composition by weight: from about 50% to about96% PbO+ZnO, from about 1% to about 18% B₂O₃, from about 2% to about 15%SiO₂, from about 1% to about 10% Al₂O₃. Additionally, up to about 5%MgO+BaO and/or up to about 10% TiO₂+Nb₂O₅+ZrO₂ may optionally beincluded therewith. However, it will be appreciated that the compositionof the glass composition can be varied, as needed, to meet theparticular requirements of the sealing operation. Higher melting pointglass frits can be used in higher temperature applications.

The one or more glass fits can be, but need not be, crystallizing glassfrits, which crystallize either before or during firing. One or moreglass fits can be of the lead bismuth borate glass type, comprising byweight from about 70% to about 96% PbO+Bi₂O₃, from about 3% to about 30%B₂O₃ and from about 1% to about 5% SiO₂.

In another embodiment, the one or more glass fits can be lead-free andcadmium-free, such as bismuth glasses comprising by weight from about50% to about 96% Bi₂O₃+ZnO, and from about 3% to about 18% B₂O₃. Apreferred variation of this embodiment also includes from about 1% toabout 15% SiO₂.

Alternatively, lead-free and cadmium free glasses can be of thealkali-tin-zinc-phosphate type, and can comprise by weight from about20% to about 55% SnO, from about 25% to about 37% ZnO, from about 10% toabout 45% P₂O₅ and from about 0.5% to about 15% Li₂O+Na₂O+K₂O+Rb₂O.

In addition to the glass frits, sealing glass compositions of thepresent invention can contain one or more fillers and/or expansionmodifiers. Examples of such fillers include cordierite(2MgO.2Al₂O₃.5SiO₂), lead titanate (PbO.TiO₂); amorphous silica, leadniobate, lead titanium niobate, beta eucryptite, zirconium phosphate,magnesium pyrophosphate.

The one or more glass frits and any optional fillers are dispersed in apolymeric binder system that exists as a waxy solid at room temperature(˜22.5° C.), but which melts at a temperature of less than about 70° C.The composition of the polymeric binder system is not per se critical,and a variety of polymeric binder systems can be used.

In some applications, however, it is important that the polymeric bindersystem completely burn out a relatively low temperature, less than 450°C., preferably less than 350° C. This is particularly important duringthe fabrication of MEMS devices. Ideally, the polymeric binder systemshould completely burn out before the one or more glass frits reachtheir glass transition temperature (Tg). In practice, some of the bindersystem may still be in the process of burning out as the one or moreglass fits reach their glass transition temperature (Tg).

A polymeric binder system suitable for use in sealing cap wafers todevice wafers in MEMS device fabrication comprises at least one C₁₄ orhigher linear primary alcohol. n-Cetyl alcohol is particularlypreferred. The polymeric binder system also preferably further comprisesacrylic polymers. The presently most preferred acrylic polymers for usein the invention are iso-butyl methacrylate polymers. Optionalcomponents can be added to the polymeric binder system as needed.Cellulose-ethers are not preferred for use in the invention inasmuchthey tend not to burn out at the relatively low processing temperaturesused in MEMS device fabrication.

The hot-melt sealing glass composition according to the invention ispreferably formed by mixing the components of the polymeric bindersystem together at a temperature slightly above the melting point of thefatty alcohol. The one or more glass frits are added to the moltenpolymeric binder system under mixing conditions and then the paste ispassed through a three roll mill heated to a temperature above about 70°C. to ensure that there are no large particles or agglomerations.

The present invention also provides a method of bonding a cap wafer to adevice wafer during MEMS device fabrication. The method comprisesheating a hot-melt sealing glass composition according to the inventionto a temperature above the melting point of the polymeric binder systembut below the temperature at which the polymeric binder system begins tosubstantially volatilize; applying the hot-melt sealing glasscomposition to a cap wafer and/or a device wafer by screen printing, padprinting, extrusion, dispensing, or any other conventional applicationmethod; positioning the cap wafer on the device wafer to encapsulate theMEMS device within a cavity formed between the cap wafer and the devicewafer; and firing the MEMS device assembly to completely burn out allorganic material in the hot-melt sealing glass composition and to meltthe one or more glass frits sufficiently to form a hermetic seal betweenthe cap wafer and the device wafer. Silicon is conventionally used fordevice wafers, but other semiconductor materials such as GaAs can alsobe used. Similarly, cap wafers can be made of silicon, othersemiconductor materials such as GaAs, or of transparent glass materialssuch as soda-lime silica glass. Firing temperatures are typically chosenin view of the components used in the MEMS device. Firing temperaturesshould be high enough to completely burn out the organic material in thehot-melt sealing glass composition and to fuse the one or more glassfrits between the cap wafer and the device wafer.

Hot-melt sealing glass compositions according to the invention areparticularly useful in the fabrication of MEMS devices. Unlikeconventional frit bonding pastes, which are solvent-based liquiddispersions at room temperature, hot-melt sealing glass compositionsaccording to the present invention are waxy solids at room temperature,which simplifies handling and storage. The polymeric binder system meltsinto a flowable liquid state at a moderately low temperature therebyallowing for the rapid application of the molten hot-melt sealing glasscomposition to substrates using screen printing, pad printing,extrusion, or other conventional application methods and equipment. Theviscosity of the molten hot-melt sealing glass composition can bedirectly controlled by adjusting the temperature (e.g., to lower theviscosity one needs merely to raise the temperature).

Hot-melt sealing glass compositions according to the invention provide asubstantial increase in the rate of MEMS device production. The moltenhot-melt sealing glass composition rapidly solidifies after applicationto the cap wafer and/or device wafer substrate and thus requires nosubsequent drying step. The solidified pattern stands up to rigorouspart handling without smudging or moving. In fact, other manufacturingprocess steps can be carried out within seconds after the moltenhot-melt sealing glass composition has been deposited on the substrate.This provides a substantial advantage over the use of conventionalsolvent-based frit bonding pastes, which are liquid dispersions at roomtemperature and which must be forced dried in ovens before additionalprocessing can be completed. The elimination of the need for drying thusincreases component production speed and further reduces component lossdue to breakage from handling.

Another advantage of the hot-melt sealing glass composition according tothe invention is that it does not release volatile organic compoundsduring component fabrication. The components of the polymeric bindersystem are non-hazardous and do not evaporate when heated at thepreferred application temperatures. Preferably, the organic materialsused in the hot-melt sealing glass composition according to theinvention do not evaporate or volatilize until they are subjected tofiring conditions, at which point the organic materials can becompletely combusted into non-hazardous products. The absence ofvolatile compounds in the composition increase the shelf life andstability of the composition as compared to conventional solvent-basedglass frit bonding pastes, which are liquid dispersions at roomtemperature.

As noted above, the hot-melt sealing glass composition according to theinvention is particularly well-suited for application to substratesusing well-known screen printing techniques among other techniques.Although conventional screen printing equipment can be used, it ispreferable for the screen, stage, and squeegee to be heated. It will beappreciated that the particular heating screen, stage, and squeegeeheating temperatures will be dictated by the melt temperature of thehot-melt sealing glass composition, the desired viscosity of the moltenmaterial; and the surface cosmetics to be achieved in the printedpattern. For example, when a hot-melt sealing glass composition is usedthat melts within the range of from about 35° C. to about 90° C., thescreen temperature will preferably be maintained within the range offrom about 40° C. to about 90° C., the stage temperature is maintainedwithin the range of from about 30° C. to about 65° C., and the squeegeetemperature is maintained up to about 65° C. By optimizing thetemperature parameters for the application equipment, the hot-meltsealing glass composition can be used to screen print, pad print,extrude, or dispense patterns having excellent print cosmetics.

The following examples are intended only to illustrate the invention andshould not be construed as imposing limitations upon the claims.

EXAMPLE 1

A glass frit (“Component 1”) and a crystallized cordierite glass ceramicfiller (“Component 2”), having the compositions shown in Table 1 belowin weight percent, were each prepared using conventional glass meltingtechniques:

TABLE 1 Component 1 Component 2 PbO 84.0 — SiO₂ 1.2 51.1 Al₂O₃ 0.8 34.0MgO — 12.9 B₂O₃ 11.0 — ZnO 3.0 — BaO — 2.0

Components 1 and 2 were milled to a fineness of D₅₀<10 microns.

EXAMPLE 2

Hot-melt Sealing Glass Composition A (“Glass A”) was formed by mixingthe various components shown in weight percent in Table 2 below in ajacketed high speed mixer at 70° C. for 1 hour:

TABLE 2 Glass A Component 1 75.6 Component 2 9.3 ALFOL ™ 16 12.1ELVACITE ™ 2045 1.5 ACRYLOID ™ B-67 1.5

ALFOL™ 16 is a 1-hexadecanol (n-cetyl alcohol) product that iscommercially available from Sasol Olefins & Surfactants GmbH (Germany).ELVACITE™ 2045 is a high molecular weight iso-butyl methacrylate polymerthat is commercially available from Lucite International Ltd (UnitedKingdom). And, ACRYLOID™ B-67 is an isobutyl methacrylate polymer thatis commercially available from Rohm and Haas (United States).

The ALFOL™ 16, ELVACITE™ 2045 and ACRYLOID™ B-67 were charged to theheated mixing chamber and well-mixed before Components 1 and 2 wereadded. After mixing, the resulting molten dispersion or paste was passedthrough a three roll mill three times and then allowed to cool to roomtemperature (˜22.5° C.).

EXAMPLE 3

Hot-melt Sealing Glass Composition A formed in Example 2 above washeated to a temperature of 65° C. and then screen printed onto a glasscap wafer using a 325 mesh screen to form a 175 microns wide sealingline. The composition rapidly solidified on the cap wafer within a fewseconds after the screen was removed. The screen printed cap wafer wasthen heated in an air atmosphere from room temperature to 295° C. at arate of 3° C./min, held at 295° C. for about 15 to 45 minutes to burnoff organics, and then heated at a rate of about 3° C./min to 410° C.and held at 410° C. for 5 minutes and then furnace cooled to roomtemperature. After this pre glazing step, the cap wafer was then placedover the device wafer in a wafer bonder after evacuating the chamber atabout 200° C. and back filling with a desired gas. Then the entireassembly was heated at about 20 to 30° C./min to a peak firingtemperature of about 450° C. for about 15 minutes, and the entireassembly was furnace cooled to room temperature in the wafer bonder. Abonding force appropriate to the wafer size was applied on the cap waferduring the bonding process to force the sealing glass to wet and bonddevice wafer to the cap wafer. During the heating cycle, the organiccomponents of the composition completely burned out before the glassfrits melted and flowed. The cap wafer was hermetically sealed to thedevice substrate.

Compositional percentages are by weight. Certain embodiments of theinvention are envisioned where at least some percentages, temperatures,times, and ranges of other values are preceded by the modifier “about.”Numerical ranges including “about” are also intended to provide supportfor the range without “about.” “Comprising” is intended to providesupport for “consisting of” and “consisting essentially of.” Numericalranges of oxides or other ingredients that are bounded by zero on thelower end (for example, 0-10 wt % TiO₂) are intended to provide supportfor the concept “up to [the upper limit],” for example “up to 10 wt %TiO₂” as well as a positive recitation that the ingredient in questionis present in an amount that does not exceed the upper limit; thephrases in this sentence are interchangeable. A recitation such as “upto 10 wt % TiO₂” is intended to provide support for “0-10 wt % TiO₂” aswell as for “comprises TiO₂, provided the amount does not exceed 10 wt%.” A recitation such as “70-96 wt % PbO+Bi₂O₃” means that any or all ofPbO and/or Bi₂O₃ may be present in an amount of 70-96 wt %.

Additional advantages and modifications will readily occur to personshaving skill in the art. Therefore, the invention in its broader aspectsis not limited to the specific details and illustrative examples shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method of bonding a cap wafer to a device wafer so as tohermetically seal and isolate a MEMS device in a cavity definedtherebetween, the method comprising: providing a hot-melt sealing glasscomposition comprising: from about 50% by weight to about 95% by weightof a glass component comprising one or more glass frits, and from about5% by weight to about 50% by weight of a polymeric binder system that isa solid at 22.5° C. but which melts at a temperature within the range offrom 35° C. to 90° C., wherein the polymeric binder system is capable ofcomplete burn-out at a temperature below 450° C., and wherein the glasscomponent is capable of melting and flowing at a temperature below 500°C. heating the hot-melt sealing glass composition above the meltingpoint of the polymeric binder system and below a temperature at whichthe polymeric binder system begins to substantially volatilize, to forma molten paste; depositing the molten paste onto the cap wafer and/orthe device wafer by screen printing; allowing the deposited molten pasteto re-solidify; positioning the cap wafer and device wafer in relationto each other such that at least the glass component of the hot-meltsealing glass composition is positioned therebetween; and firing the capwafer and device wafer at a temperature above the melting point of theglass component to form a hermetic seal between the cap wafer and devicewafer that isolates the MEMS device in the cavity defined therebetween.2. The method of claim 1, wherein the glass component comprises: (a)from about 50 to about 96 wt % PbO+ZnO, (b) from about 1 to about 18 wt% B₂O₃, (c) from about 2 to about 15 wt % SiO₂, and (d) from about 1 toabout 10 wt % Al₂O₃.
 3. The method of claim 2, wherein the glasscomponent further comprises at least one of (a) and (b): (a) up to about5 wt % MgO+BaO and (b) up to about 10 wt % TiO₂+Nb₂O₅+ZrO₂.
 4. Themethod of claim 1, wherein the glass component comprises: (a) from about70 to about 96 wt % PbO+Bi₂O₃, (b) from about 3 to about 30 wt % B₂O₃,and (c) from about 1 to about 5 wt % SiO₂.
 5. The method of claim 1,wherein the glass component comprises: (a) from about 50 to about 96 wt% Bi₂O₃+ZnO and (b) from about 1 to about 30 wt % B₂O₃.
 6. The method ofclaim 1, wherein the glass component is lead-free and cadmium-free, andcomprises: (a) from about 20 to about 55 wt % SnO, (b) from about 25 toabout 37 wt % ZnO, (c) from about 10 to about 45 wt % P₂O₅, and (d) fromabout 0.5 to about 15 wt % Li₂O+Na₂O+K₂O+Rb₂O.
 7. The method of claim 1,wherein the polymeric binder comprises a C14 or higher linear primaryalcohol.
 8. The method of claim 1, wherein the polymeric bindercomprises n-cetyl alcohol.
 9. The method of claim 1, wherein thepolymeric binder further comprises acrylic polymers.
 10. The method ofclaim 9, wherein the acrylic polymers include isobutyl methacrylatepolymers.
 11. The method of claim 1, wherein the average particle sizeof the one or more glass frits in the glass component is from 3micronsto 10 microns.
 12. The method of claim 1, wherein the hot melt sealingglass composition does not contain cellulose-ethers.
 13. The method ofclaim 1, wherein the device wafer is formed of silicon or GaAs.
 14. Themethod of claim 1, wherein the cap wafer is formed of silicon, GaAs orsoda-lime silica glass.
 15. The method of claim 1, wherein the hot meltsealing glass composition does not release volatile organic compoundsduring the heating, depositing, allowing and positioning steps.
 16. Themethod of claim 1, wherein organic compounds present in the hot meltsealing composition do not evaporate or volatilize until the firingstep.
 17. The method of claim 1, further comprising the step ofsubjecting the device wafer to further processing and handling betweenthe allowing step and the positioning step.
 18. The method of claim 1,further comprising, between the allowing step and the positioning step,a pre glazing step in which the device wafer and/or the cap wafer onwhich the hot melt sealing composition has re-solidified is heated to apre glazing temperature and then allowed to cool.
 19. The method ofclaim 18, further comprising, between the positioning step and thefiring step, a back filling step in which the cavity between cap waferand the device wafer is filled with a desired gas.